It is common practice to connect the mostly probe-shaped starting electrode with the remote main electrode through a resistance for ignition-current limitation. This may lead to an attack on the seals of both the starting electrode and adjoining main electrode in the discharge vessel to electrolysis. U.S. Pat. No. 2,508,114 accordingly provides a thermal switch in the ignition circuit in series with the starting resistance which interrupts the conductive connection between the starting electrode and the remote main electrode during operation of the lamp. German Pat. No. 1,207,006 on the other hand discloses the elimination of sputtering of the electrode material caused by a continuously flowing current in the ignition circuit. Instead of providing a current-limiting resistance between the starting electrode and the remote main electrode, there is inserted a glow lamp which extinguishes after ignition of the lamp and thus interrupts the conductive connection.
The phenomenon of electrolysis is particularly intense in high pressure mercury vapor discharge lamps comprising a starting electrode and having metal halide additives. German Pat. No. 1,217,496 therefore discloses the insertion of a thermal switch which in normal operation of the lamp provides conductive connection of the starting electrode with the adjoining main electrode. During this, the starting electrode may either remain connected with the remote main electrode through a high-ohmic starting resistance, or it may be disconnected from the main electrode.
In all these known circuit arrangements there exists no conductive connection whatsoever between the starting electrode and its adjoining main electrode during the starting operation. According to U.S. Pat. No. 2,508,114 and German Pat. No. 1,207,006, the connection between the starting electrode and the remote main electrode is interrupted subsequent to ignition of the lamp, i.e. not before operation of the lamp and, consequently, the starting electrode is on free potential. German Pat. No. 1,217,496 on the other hand discloses a device wherein, not before the lamp has ignited, however, the starting electrode is conductively connected to the adjoining main electrode, i.e. these electrodes then have the same potential.
Moreover, it is known (German Pat. No. 1,589,286) to connect a glow switch parallel to the main electrodes of a high pressure discharge lamp having no auxiliary starting electrode, in series with a resistance and a capacitor. Thus, a high sequence of impact-voltage pulses initiated between the main electrodes may already lead to lower ignition voltage peaks and bring about satisfactory ignition.